Add tutorial for presolver plugin to CHANGELOG

Author: fvz185

CHANGELOG.md

@@ -18,6 +18,7 @@
 - Added isActive() which wraps SCIPvarIsActive() and test
 - Added aggregateVars() and tests
 - Added example shiftbound.py
+- Added a tutorial in ./docs on the presolver plugin
 ### Fixed
 - Raised an error when an expression is used when a variable is required
 - Fixed some compile warnings